A liquid crystal display (LCD) is a display device for implementing a desired image by selectively transmitting light emitted from a backlight using a polarization phenomenon of liquid crystal.
FIG. 1 shows a phenomenon for selectively transmitting light in a TN-LCD used widely. As shown in FIG. 1A, an angle between the polarization axes between a light source side polarizer plate 10 and a viewer side polarizer plate 20 is 90°. Nematic liquid crystal molecules 30 are filled between the two polarizer plates and the liquid crystal molecules 30, which are close to the polarizer plates, are aligned in the same directions as the polarization axes of the polarizer plates. Accordingly, the alignment directions of the liquid crystal molecules which are close to the two polarizer plates are orthogonal to each other, similar to the polarizer plates. In addition, the liquid crystal molecules in a middle portion are twisted as shown in the drawing.
Accordingly, light 40 polarized by the light source side polarizer plate rotates by 90° by the nematic liquid crystal molecules and, as a result, passes through the viewer side polarizer plate.
However, when an electrical field is applied to the liquid crystal molecules 30, the liquid crystal molecules 30 are arranged parallel to the direction of the electrical field as shown in FIG. 1b. Accordingly, the light 40 which passes through the light source side polarizer plate 10 does not rotate and reaches the viewer side polarizer plate 20 in the direction polarized by the light source side polarizer plate such that the light does not transmit through the viewer side polarizer plate 20 which rotates by 90° with respect to the light source side polarizer plate 10.
The LCD controls the transmission and blocking of the light by selectively applying the electrical field to each pixel using the above-described phenomenon.
In the LCD, the contrast characteristic is improved when the light is surely transmitted and blocked such that a bright state and a dark state appear. In particular, when light leakage is minimized in the dark state, it is possible to improve the contrast characteristic. No problem is caused in the contrast characteristic when a display device is viewed from the front side, but the light leakage is unlikely to be perfectly prevented in the dark state when the display device is viewed at an inclination angle. As described above, in the LCD, the light is blocked by providing a polarizer plate having a transmission axis perpendicular to the light polarized linearly at the viewer side using a polarization phenomenon. However, when the light which reaches the viewer side polarizer plate is not perfectly linearly polarized, a portion of the light may be leaked.
Such a phenomenon may occur when the light travels in an inclination direction (in the viewer's direction), not in a vertical direction. That is, the dark state is perfectly implemented when the nematic liquid crystal molecules are aligned parallel to the traveling direction of the light as shown in FIG. 1b. Accordingly, since the alignment direction of the liquid crystal molecules and the traveling direction of the light are not perfectly parallel to each other when the light travels in the inclination direction, an additional phase variation occurs and thus the light is not perfectly blocked.
Another reason why the light leakage occurs in the dark state is because the liquid crystal molecules are aligned in a horizontal direction equal to the orientation of an alignment film, not parallel to the direction of the electrical field, in the boundary between the alignment film and the liquid crystal (that is, the polarizer plate side), and become aligned parallel to the direction of the electrical field as getting away from the boundary between the alignment film and the liquid crystal, as shown in FIG. 2. Accordingly, there is caused a so-called splay alignment that the inclination angles of the liquid crystal molecules gradually increase to be close to 90 from the boundary between the alignment film and the liquid crystal to the center of the liquid crystal layer.
The light may not be perfectly blocked in the dark state by the splay alignment of the liquid crystal layer.
Conventionally, a variety of technologies were suggested in order to prevent the light leakage. For example, Korean Patent Publication No. 10-0376378 discloses the technology of improving the gray-scale characteristic and reducing light leakage using an O film. In this technology, variations in light leakage amount among gray scales according to viewing angles are minimized by an O-plate compensator having an inclination angle. In this technology, since light leakage amount can be properly controlled according to the viewing angles, it is possible to manufacture a LCD having a high gray-scale characteristic regardless of the viewing angle.
However, in this technology, since the O film is directly formed on a glass substrate, productivity deteriorates. That is, in order to manufacture the O-plate compensator, a low pretilt alignment layer and high pretilt alignment layer are formed on the glass substrate. In this case, it takes much time to perform the process of forming the layers on the glass substrate and it is difficult to manage the substrate.
In addition, in the conventional method, light leakage which occurs by the splay alignment of the liquid crystal molecules can be reduced. However, the light leakage due to the +C alignment of liquid crystal molecules (that is, since the liquid crystal molecules are aligned parallel to the direction of the electrical field, the light can be prevented from leaking to the front side. However, when the light travels in an inclination direction, a retardation occurs and thus a perfect dark state cannot appear) is unlikely to be reduced.
When light reaches the polarizer plate at an inclination angle, a slight phase retardation occurs in the polarizer plate, thereby causing light leakage. However, in the Korean Patent Publication No. 10-0376378, it is difficult to perfectly prevent light from leaking due to such various causes.